In the fabrication of various EEPROM devices, it is frequently necessary to utilize etch processes with high selectivity. In particular, various gate structures (e.g., floating gate, control gate, erase gate) must typically be precisely defined by various etch processes without risk of damage to these gate structures. One process for floating gate definition involves the formation of two spacers, an undoped densified silicon dioxide spacer beneath a phosphorous doped silicon dioxide spacer. The phosphorous doped spacer is then removed by a wet etch after the gate is defined.
One etch process previously employed in FET fabrication utilizes a mixture of ammonium hydroxide and hydrogen peroxide, which is also called "ammonium peroxide" or NH.sub.4 OH/H.sub.2 O.sub.2, to smooth a window opened in a composite dielectric comprising BPTEOS over TEOS on a crystalline silicon substrate. After the window is opened with dry etching procedures, thereby exposing bare crystalline silicon, the wafer is exposed to an ammonium peroxide cleaning solution to remove sodium or other contaminants from the surface of the BPTEOS. (The cleaning solution utilizes approximately eight parts H.sub.2 O, two parts H.sub.2 O.sub.2 [30% concentrated] and one part concentrated NH.sub.4 OH.) It has been observed that the ammonium peroxide solution isotropically etches the BPTEOS somewhat and does not significantly etch the underlying TEOS or damage the crystalline silicon substrate.